Open-loop MIMO diversity coding techniques are widely adopted in wireless communication systems to enhance transmission reliability. Orthogonal transmit diversity (OTD) and orthogonal space-time (OST) coding are two conventional approaches of particular effectiveness. Hybrid combinations of both are also possible.
Frequency hopping, which is utilized in both LTE and GSM systems, is a primary example of an OTD coding technique. Another OTD coding example can be found in the antenna switching option of the LTE uplink.
Regarding OST, for two transmit antenna ports, the OST coding approach is adopted as the space-frequency block coding (SFBC) in the LTE system. More specifically, the transmitted signal is based on the following encoding matrix taken from 3GPP TS 36.211 V8.6.0 (200903):
  2  ⁢          ⁢  TX  ⁢          ⁢  scheme  ⁢          ⁢      (          S      ⁢                          ⁢      F      ⁢                          ⁢      B      ⁢                          ⁢      C        )    ⁢            :        ⁢                  [                                        X            1                                                X            2                                                            X            2            ′                                                -                          X              3              ′                                            ]  where {X1,X2} are input symbols. Based on the orthogonal design theory described by V. Tarokh et al. in “Space-time block codes from orthogonal designs,” IEEE Transactions on Information Theory, vol. 45, no. 5, July 1999, this SFBC coding scheme can fully exploit transmit diversity from the two transmit antennas regardless of the underlying modulation and coding scheme (MCS) adopted for the input symbols. This SFBC coding scheme is also known as the Alamouti scheme based on the paper by S. V. Alamouti entitled, “A simple transmit diversity technique for wireless communications,” IEEE J. Sel. Areas Comm., vol. 16, no. 8, pp. 1451-1458, October 1998. In practical terms, the diversity benefits lead to steeper Block Error Rate (BLER) or Bit Error Rate (BER) curves when compared to those for conventional single transmit antenna transmission.
However, for systems with more than two transmit antennas, the theory also concludes that such full-diversity coding does not exist. Hence, for the case of four transmit antenna ports, a hybrid of the OST and OTD coding approaches is adopted in the LTE system. More specifically, the transmitted signal is based on the following encoding matrix also taken from 3GPP TS 36.211 V8.6.0 (2009-03):
  4  ⁢          ⁢  TX  ⁢          ⁢  scheme  ⁢          ⁢      (                  S        ⁢                                  ⁢        F        ⁢                                  ⁢        B        ⁢                                  ⁢        C            +              T        ⁢                                  ⁢        S        ⁢                                  ⁢        F        ⁢                                  ⁢        D              )    ⁢            :        ⁢                  [                                        X            1                                                X            2                                    0                          0                                      0                          0                                      X            3                                                X            4                                                            X            2            ′                                                -                          X              1              ′                                                0                          0                                      0                          0                                      X            4            ′                                                -                          X              3              ′                                            ]  where {X1, X2, X3, X4} are input symbols.
Several weaknesses of the open-loop MIMO coding schemes in LTE have been identified. The receiver of an OST-coded signal relies on perfect cancellation of the cross-interference to achieve the desired transmission performance and reliability. However, two practical factors constrain the achievability of perfect cross-interference cancellation.                First, in low signal-to-noise ratio (SNR) scenarios, channel estimation quality is generally lower. With these non-ideal channel estimates as inputs, an OST receiver leaves a significant amount of cross-interference unaffected in the received signal, which negatively impacts the performance and reliability of the open-loop MIMO transmission.        Second, perfect cross-interference cancellation also requires the actual channel coefficients experienced by the multiple-input symbols to be identical across certain frequency or time intervals. These requirements are not met for highly dispersive channels or for high mobility scenarios. For these cases, the open-loop MIMO transmission performance and reliability is compromised.        
From extensive performance analysis, it is found that the performance improvement of the 4TX scheme over the 2TX scheme is not significant even though the transmitter hardware is doubled. Therefore, the hybrid combination of the OST and OTD coding schemes is not very effective in exploiting the full benefits of the extra transmission hardware.
It is also noted that the 4TX scheme in LTE carries the same data throughput as the 2TX scheme. That is, the transmission rates do not explicitly scale up with the amount transmission hardware. In addition, the total transmit power has to be shared by several transmit antennas in the LTE open-loop schemes. This can cause lower power amplifier efficiency and can potentially limit the uplink coverage.